The long or medial s (ſ) is a form of the minuscule letter s that was formerly used when the s occurred within or at the beginning of the word, for example ſinfulneſs ("sinfulness"). The modern letterform was called the terminal or short s.

The medial s was subject to confusion with the minuscule f, sometimes even having an f-like nub appended to its middle. The practice later died out in Roman and Italic faces by the end of the 19th century. It survived longer in the German Fraktur; the modern German letter ß (ess-tsett) is a ligature representing ſs, a fact more obvious in some typefaces than in others.

The medial s is represented in Unicode by the hexadecimal value 17F, and may be represented in HTML as &#383;.

The confusion between the medial s and f has been the subject of some intentional humour, much of it involving phrases like "sucking pig", and forms the basis of Benny Hill's song "Fad-Eyed Fal" (i.e., Sad-Eyed Sal)